Nanoparticles are nanometer-sized materials e.g., metals, semiconductors, polymers, and the like, possessing unique characteristics because of their small size. Nanoparticles in both aqueous and non-aqueous solvents can be synthesized using a variety of methods.
The conformation of a polymer in solution is dictated by various conditions of the solution, including its interaction with the solvent, its concentration, and the concentration of other species that may be present. The polymer can undergo conformational changes depending on the pH, ionic strength, cross-linking agents, temperature and concentration. For polyelectrolytes, at high charge density, e.g., when “monomer” units of the polymer are fully charged, an extended conformation is adopted due to electrostatic repulsion between similarly charged monomer units. Decreasing the charge density of the polymer, either through addition of salts or a change of pH, can result in a transition of extended polymer chains to a more tightly-packed globular i.e. collapsed conformation. The collapse transition is driven by attractive interactions between the polymer segments that override the electrostatic repulsion forces at sufficiently small charge densities. A similar transition can be induced by changing the solvent environment of the polymer. This collapsed polymer is itself of nanoscale dimensions and is, itself, a nanoparticle. In this specification and claims the term “collapsed polymer” refers to an approximately globular form, generally as a spheroid, but also as an elongate or multi-lobed conformation collapsed polymer having nanometer dimensions. This collapsed conformation can be rendered irreversible by the formation of intramolecular chemical bonds between segments of the collapsed polymer, i.e. by cross-linking.
Macromolecules, i.e. polymers with the appropriate functional groups can undergo inter- or intra-molecular cross-linking reactions to produce new materials or new molecules with distinct properties, such as for example, shape, solubility, thermal stability, and density. These reactions are important in making new materials and various schemes for chemical reactions leading to cross-linking are described in the literature. For example, U.S. Pat. No. 5,783,626—Taylor et al, issued Jul. 21, 1998, describes a chemical method to cross-link allyl-functional polymers in the form of latexes, containing enamine moieties and pendant methacrylate groups via a free-radical cross-linking reaction during film formation producing coatings with superior solvent resistance and increased thermal stability. Polymer cross-linking has also been used to stabilize semiconductor and metal nanoparticles. U.S. Pat. No. 6,872,450—Liu et al, issued Mar. 29, 2005, teaches a method for stabilizing surface-coated semiconductor nanoparticles by self assembling diblock polymers on the surface coating and cross-linking the functional groups on the diblock polymer. Similarly, U.S. Pat. No. 6,649,138—Adams et al, issued Nov. 18, 2003, describes how branched amphipathic dispersants coated onto hydrophobic nanoparticles can also be cross-linked to form a permanent cohesive over coating around the nanoparticle.
Chemical means of cross-linking can be through radical reactions of pendant groups containing unsaturated bonds as described in aforesaid U.S. Pat. No. 5,783,626. Another method is through the use of molecules having multifunctional groups than can react with the functional groups of the polymer as described in aforesaid United States U.S. Pat. Nos. 6,649,138 and 6,872,450. Alternatively, cross-linking can be achieved though high energy radiation, such as gamma radiation. The most common method of preparing chalcogenide semiconductor nanocrystals is the TOP/TOPO synthesis (C. B. Murray, D. J. Norris, and M. G. Bawendi, “Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites,” J. Am. Chem. Soc., 115:8706-8715, 1993). However, this method again involves multiple chemical steps and large volumes of expensive and toxic organometallic metal precursors and organic solvents. Furthermore, such nanoparticles need to be chemically modified in order to render them soluble in aqueous solution, which is important for a number of applications. Chalcogenide nanoparticles have also been synthesized in aqueous solution at low temperature using water-soluble thiols as stabilizing agents ((a) Rajh, O. L. Mićić, and A. J. Nozik, “Synthesis and Characterization of Surface-Modified Colloidal CdTe Quantum Dots,” J. Phys. Chem., 97: 11999-12003, 1993. (b) A. L. Rogach, L. Ktsikas, A. Kornowski, D. Su, A. Eychmüller, and H. Weller, “Synthesis and Characterization of Thiol-Stabilized CdTe Nanocrystals,” Ber. Bunsenges. Phys. Chem., 100(11): 1772-1778, 1996. (c) A. Rogach, S. Kershaw, M. Burt, M. Harrison, A. Kornowski, A. Eychmüller, and H. Weller, “Colloidally Prepared HgTe Nanocrystals with Strong Room-Temperature Infrared Luminescence,” Adv. Mater. 11:552-555, 1999. (d) Gaponik, N., D. V. Talapin, A. L. Rogach, K. Hoppe, E. V. Shevchencko, A. Kornowski, A. Eychmüller, H. Weller, “Thiol-capping of CdTe nanocrystals: an alternative to organometallic synthetic routes,” Journal of Physical Chemistry B, 2002, vol. 106, iss. 39, p. 7177-7185. (e) A. L. Rogach, A. Kornowski, M. Gao, A. Eychmüller, and H. Weller, “Synthesis and Characterization of a Size Series of Extremely Small Thiol-Stabilized CdSe Nanocrystals,” J. Phys. Chem. B. 103:3065-3069, 1999). However, this method generally requires the use of an inert atmosphere with multiple processing steps and production of precursor gases. Another water-based synthesis involves the formation of undesirable by-products that must first be removed before semiconductor particles can be obtained (H. Zhnag, Z. Hou, B. Yang, and M. Gao, “The Influence of Carboxyl Groups on the Photoluminescence of Mercaptocarboxylic Acid-Stabilized Nanoparticles,” J. Phys. Chem. B, 107:8-13, 2003).
CdTe nanocrystals are known to have tunable luminescence from green to red and have shown tremendous potential in light-emitting thin films (A. A. Mamedov, A. Belov, M. Giersig, N. N. Mamedova, and N. A. Kotov, “Nanorainbows: Graded Semiconductor Films from Quantum Dots,” J. Am. Chem. Soc., 123: 7738-7739, 2001), photonic crystals (A. Rogach, A. Susha, F. Caruso, G. Sukhoukov, A. Kornowski, S. Kershaw, H. Möhwald, A. Eychmüller, and H. Weller, “Nano- and Microengineering: Three-Dimensional Colloidal Photonic Crystals Prepared from Submicrometer-Sized Polystyrene Latex Spheres Pre-Coated with Luminescent Polyelectrolyte/Nanocrystal Shells,” Adv. Mater. 12:333-337, 2000), and biological applications (N. N. Memedova and N. A. Kotov, “Albumin-CdTe Nanoparticle Bioconjugates: Preparation, Structure, and Interunit Energy Transfer with Antenna Effect,” Nano Lett., 1(6):281-286, 2001). PbTe and HgTe materials exhibit tunable emission in the infrared and look promising in the telecommunications industry. HgTe nanoparticles have been incorporated into more sophisticated assemblies, particularly as components in thin-film electroluminescent devices ((a) A. L. Rogach, D. S. Koktysh, M. Harrison, and N. A. Kotov, “Layer-by-Layer Assembled Films of HgTe Nanocrystals with Strong Infrared Emission,” Chem. Mater., 12:1526-1528, 2000. (b) É. O'Conno, A. O'Riordan, H. Doyle, S. Moynihan, a. Cuddihy, and G. Redmond, “Near-Infrared Electroluminescent Devices Based on Colloidal HgTe Quantum Dot Arrays,” Appl. Phys. Lett., 86: 201114-1-20114-3, 2005. (c) M. V. Kovalenko, E. Kaufmann, D. Pachinger, J. Roither, M. Huber, J. Stang, G. Hesser, F. Schäffler, and W. Heiss, “Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies: From Telecommunications to Molecular Vibrations,” J. Am. Chem. Soc., 128:3516-3517, 2006) or solar cells (S. Günes, H. Neugebauer, N. S. Sariiciftci, J. Roither, M. Kovalenko, G. Pillwein, and W. Heiss, “Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes,” Adv. Funct. Mater. 16:1095-1099, 2006). PbTe, on the other hand, can be grown in a variety of glasses at high temperatures to produce composite materials for applications in optoelectronic devices ((a) A. F. Craievich, O. L. Alves, and L. C. Barbosa, “Formation and Growth of Semiconductor PbTe Nanocrystals in a Borosilicate Glass Matrix,” J. Appl. Cryst., 30:623-627, 1997. (b) V. C. S. Reynoso, A. M. de Paula, R. F. Cuevas, J. A. Medeiros Neto, O. L. Alves, C. L. Cesar, and L. C. Barbosa, “PbTe Quantum Dot Doped Glasses with Absorption Edge in the 1.5 μm Wavelength Region,” Electron. Lett., 31(12):1013-1015, 1995).
Doping of CdTe with Hg results in the formation of CdHgTe composite nanocrystals. Red shifts in absorbance/photoluminescence spectra and enhanced PL are observed with increasing Hg content (A. L. Rogach, M. T. Harrison, S. V. Kershaw, A. Kornowski, M. G. Burt, A. Eychmüller, and H. Weller, “Colloidally Prepared CdHgTe and HgTe Quantum Dots with Strong Near-Infrared Luminescence,”phys. scat. sol., 224(1):153-158, 2001). Cd1-xHgxTe alloys are popular components in devices used for near-IR detector technology. A variety of methods have been developed to create these materials. U.S. Pat. No. 7,026,228—Hails et al, issued Apr. 11, 2006, describes an approach to fabricating devices and semiconductor layers of HgCdTe in a metal organic vapour phase epitaxy (MOVPE) process with mercury vapor and volatile organotelluride and organocadmium compounds. In a different approach, U.S. Pat. No. 7,060,243—Bawendi et al, issued Jun. 13, 2006, describes the synthesis of tellurium-containing nanocrystals (CdTe, ZnTe, MgTe, HgTe and their alloys) by the injection of organometallic precursor materials into organic solvents (TOP/TOPO) at high temperatures. U.S. Pat. No. 6,126,740—Schulz, issued Oct. 3, 2000, discloses another non-aqueous method of preparing mixed-semiconductor nanoparticles from the reaction between a metal salt and chalcogenide salt in an organic solvent in the presence of a volatile capping agent.
Mixtures of CdTe and PbTe have also been investigated for IR detection in the spectral range of 3 to 5 μm. However, because these materials have such fundamentally different structures and properties (S. Movchan, F. Sizov, V. Tetyorkin. “Photosensitive Heterostructures CdTe—PbTe Prepared by Hot-Wall Technique,” Semiconductor Physics, Quantum Electronics & Optoelectronics. 2:84-87, 1999. V), the preparation of the alloy is extremely difficult. U.S. Pat. No. 5,448,098—Shinohara et al, issued Sep. 5, 1995, describes a superconductive device based on photo-conductive ternary semiconductors such as PbCdTe or PbSnTe. Doping of telluride quantum dots, e.g. CdTe, with transition metals, e.g. Mn offers the possibility of combining optical and magnetic properties in one single nanoparticle ((a) S. Mackowski, T. Gurung, H. E. Jackson, L. M. Smith, G. Karczewski, and J. Kossut, “Exciton-Controlled Magnetization in Single Magnetic Quantum Dots,”Appl. Phys. Lett. 87: 072502-1-072502-3, 2005. (b) T. Kümmel, G. Bacher, M. K. Welsch, D. Eisert, A. Forchel, B. König, Ch. Becker, W. Ossau, and G. Landwehr, “Semimagnetic (Cd,Mn)Te Single Quantum Dots—Technological Access and Optical Spectroscopy,” J. Cryst. Growth, 214/215:150-153, 2000). Unfortunately, these materials are mostly fabricated using thin-film technologies such as molecular beam epitaxy or chemical vapour deposition and the necessity for a very controlled environment during growth makes these materials inaccessible. Some mixed-metal tellurides such as CdHgTe (S. V. Kershaw, M. Burt, M. Harrison, A. Rogach, H. Weller, and A. Eychmüller, “Colloidal CdTe/HgTe Quantum Dots with High Photoluminescence quantum Efficiency at Room Temperature,” Appl. Phys. Lett., 75: 1694-1696, 1999); and CdMnTe (N. Y. Morgan, S. English, W. Chen, V. Chemornordik, A. Russ, P. D. Smith, A. Gandjbakhche, “Real Time In Vivo Non-Invasive Optical Imaging Using Near-Infrared Fluorescent Quantum Dots,” Acad. Radiol, 12(3): 313-323, 2005) quantum dots have been prepared in aqueous solution which is an adaptation of the synthetic technique outlined in supra Rajh, O. L. et al. However, all of the aforementioned methods involve many processing steps, sophisticated equipment or large volumes of expensive and toxic organometallic metal precursors and organic solvents.
A simple tellurite reduction method to prepare cadmium telluride materials has been used using sodium tellurite (Na2TEO3) as a tellurium precursor salt with a suitable reducing agent, such as NaBH4 with My+ cations (H. Bao, E. Wang, and S. Dong, “One-Pot Synthesis of CdTe Nanocrytals and Shape Control of Luminescent CdTe-Cystine Nanocomposites,” small, 2(4):476-480, 2006).
Accordingly, there is a need in the art for an environmentally friendly, “one-pot”, cost-effective, and generalizable method of directly producing metallic, metallic alloyed, semiconductor, oxide, and other forms of nanocomposite particles having effective functionality in a multitude of scientific disciplines.